1. Field of the Invention
The invention relates to antennas for sending and receiving circularly polarized signals and to phase shifters and methods phase shifting for circularly polarized signals for use with such antennas. In particular, the invention relates to a low cost apparatus and method of phase shifting, and an antenna in combination with such method and apparatus.
2. Background of the Invention
Circularly polarized signals are well known, being a composition of two orthogonally polarized waves of equal frequency, in equal magnitude, propagating in phase offset. A common phase offset is 90.degree. usually designated phase quadrature. The polarization will be either right-handed or left-handed depending on the relative sense of the resultant circularly polarized wave as produced by two phase offset orthogonal waves. In a circularly polarized signal at any point in a given cycle, the resultant energy of the wave will be the resultant of the combined energy of the horizontal component, and the vertical component.
As horizontal and vertical energy components represent components of a sinusoidal wave the resultant energy component is constant, thus providing a circular locus for the resultant energy. While sweeping out a circle, the resultant energy moves forward at the velocity of propagation, which defines a helix about the propagation axis. See Bekowitz, Basic Microwaves, Hayden Book Company, Inc., New York 1966.
When receiving or transmitting circularly polarized signals it is necessary to phase shift the signals, either to produce the phase shift when transmitting or to eliminate it when receiving. Phase shifting may be accomplished in a number of ways, however, in the present discussion only the technique of adjusting the physical length of the transmission line is relevant. See White, Microwave Semiconductor Engineering, Van Nostrand Reinhold Co., 1982.
It is well accepted that the theory of reciprocity applies to antenna theory; meaning that an antenna can be seen and analyzed as being in either transmitting or receiving mode. Most commonly, a discussion of antenna operation speaks in the transmitting mode. The presently preferred application of the invention is in the receiving mode and therefore the description speaks primarily in the receiving mode. The invention is equally applicable to both receiving and transmitting modes.
The particular application of the presently preferred mode of practicing the invention is for receiving circularly polarized signals from the satellite system known as Global Positioning System (GPS). The GPS satellites broadcast in two frequencies, the L1 frequency at 1575.42 MHZ and the L2 frequency at 1227.6 MHZ. in one of the present preferred embodiments, the technique is applied to the L1 frequency. In another embodiment the invention is applied to a dual frequency mode, in which case the second frequency can be the GPS L2 frequency.
In the past, phase shifting for GPS receiving antennas has been accomplished by adjusting the length of the antenna elements during the manufacturing process while the antenna is attached to a test instrument. This technique which must be performed by an assembler, is expensive and time consuming and results in antennas of varying specifications.
Prior art quadrifilar helix antennas are based on the same principle as described herein for the present invention, i.e. four antenna elements are driven in quadrature phase sequence by a phase shift network. But prior phase shift network configurations are considerably more complicated and expensive to manufacture than that described for the present invention. Typically, the prior quadrifilar elements are driven from the top of the assembly, with a coaxial phase sequencer/transformer made from rigid coax elements bringing up the signals from the bottom of the structure to the top. Most currently manufactured quadrifilar antennas are built this way, and their complexity is reflected in their relatively high cost. Further increase in manufacturing cost is caused by the difficulty of maintaining tight dimensional tolerances of the phase sequencer structure. Antenna manufacturers typically work around the dimensional accuracy problem by custom tuning the individual element length.
Phase shifting requires high precision, particularly in the length of the phase shifting line. For example, 1.degree. of phase at the GPS L1 frequency having a wavelength of 19.04 centimeters is 1/2 millimeter. In other words, to obtain 2.degree. of phase accuracy, the phase shift lines must have a precision of 1 millimeter. By prior methods of manual adjustment for the length of the antenna phase shift element, precise expensive manual labor is required. The present invention provides the high precision required, in a low cost microstrip or stripline printed circuit. Further, the simplicity provided by the network reduces space requirements and cost.